Star formation? All a bit of a wind up

UK astronomers have discovered that the material flowing out of newborn stars contains a coiled, spring-shaped magnetic field.

The discovery, reported in the 1 November edition of Nature helps to explain why new stars are able to form as they condense from spinning clouds of interstellar gas.

"Astronomers know that stars form when interstellar gas and dust collapse under the influence of gravity, but these objects are rotating," explains Dr Antonio Chrysostomou, currently on leave from the University of Hertfordshire to work as associate director of the James Clerk Maxwell Telescope in Hawaii.

"The problem is that as a rotating body gets smaller it will spin up so fast that these clouds should fly apart before they have a chance to form stars - and yet we know that stars do form. Somehow, that angular momentum, or spin, needs to be removed before a star forms."

Many new stars, as they are forming, expel material from their poles at extremely high speeds. Astronomers have long supposed that these jets carried away angular momentum from forming stars, preventing them from spinning faster and faster as they condensed.

The theory is that as the gas collapses to form the spinning protostar, the interstellar gas twists the magnetic field that permeates the universe, winding it up like a corkscrew. Gas that is spinning too fast spins out along the field lines creating the polar jets observed on so many new stars.

But until now there was no real evidence to support this explanation for the process, the Hertfordshire team says. Using the Anglo-Australian Telescope (AAT) in New South Wales, Australia, Dr Chrysostomou and his team measured the circular polarisation of near-infrared radiation from a young star's jet. They were then able to reconstruct the structure of the magnetic field in the outflow.

Dr Phil Lucas, lecturer at the University of Hertfordshire, commented: "When we combine our observations with sophisticated computer modelling techniques, we were able to show that the shape of magnetic field that could reproduce our observations could only be helical." ®